1. Field of the Invention
The present invention relates to a process for reducing the production of fly ash in a wet slagging boiler. More specifically, the invention relates to melting substantially all of the fly ash and having it flow out of the furnace with the bottom ash.
2. Description of the Prior Art
In slagging furnaces it is desirable to increase the fraction of the ash which leaves the system as slag and decrease the fraction of the ash which leaves the furnace as fly ash. The reasons for increasing the fraction of ash removed as slag include: 1) the material is physically more stable than fly ash, 2) it is more dense than fly ash, 3) it is more marketable than fly ash, and 4) it does not "fly" through the boiler causing erosion.
Slag is more stable than fly ash. This is because slag is usually broken into pieces of from 1/2 inch diameter to 1/16 inch diameter, while fly ash typically has dimensions of 1/500 of an inch and less and will blow away as it is collected. For the same reason, water soluble materials are much more readily leached from fly ash. The smaller fly ash particles have a much higher surface area to volume ratio, and much more surface area is available for contact with water which leaches out water soluble materials (including small amounts of toxic metals) from the ash particle. For this reason, slag will almost always be regarded as a nonhazardous waste while fly ash of the same composition may be a hazardous waste.
The increased density of slag means that a greater weight of slag may be stored in the same volume or disposed of in the same landfill volume when compared to fly ash. Additionally, the slag will almost always be a stable fill while the fly ash might not be stable.
Fly ash has only a limited marketability. While it is useful as an extender for Portland cement and for concrete, only about 10% of the fly ash produced in the United States finds any market. Without a market, it must be disposed of at some expense. Slag is useful as an aggregate for concrete in various uses. It is useful as road bed material and as road surface material for certain applications. It is used as the aggregate in asphalt shingles. It is useful as blasting material for cleaning metal objects, rock or masonry objects. Notable among these blasting operations is the cleaning of ships. When slag replaces sand in "sand blasting" the risk for silicosis is greatly reduced. As a result of having these various uses and the fact that there is a limited supply of slag, about 75% of the boiler slag produced in the United States is sold for commercial use. The slag that is not sold is more easily disposed of than fly ash.
When fly ash is transmitted into molten slag, it is drained from the furnace. Fly ash, however, is swept through the furnace, the convective passes, super-heaters, steam reheaters, economizers and the air heater as dust, which erodes these components. While the erosion is sometimes slow enough to be harmless, it can be so rapid as to be catastrophic. Various techniques are practiced to reduce this erosion. Shields may be placed in front of tubes, or tube spacings may be increased and/or areas opened up to decrease particle velocity. Alternatively, the tubes may be constructed of specialty metals or have ceramic-type coatings installed thereon.
In addition to erosion, the ash builds up on surfaces, which reduces heat transfer and restricts gas flow. This buildup is often removed by the use of a soot blower. These soot blowers, however, are expensive to purchase, operate and maintain and at times may cause erosion themselves. It is thus more desirable to produce molten slag which is then quenched in water rather than producing fly ash.
The art has attempted to recycle fly ash in wet bottom furnaces. In this type of furnace, coal is burned and part of the ash fuses and runs from the furnace bottom as a liquid slag. The molten slag falls from the bottom of the furnace into water where it is quenched. The ratio between fly ash and bottom slag depends upon design and operating parameters and coal and ash characteristics. Cyclone fired boilers and some pulverized coal fired boilers have wet bottom furnaces, which drain the molten slag. Recognizing that some fly ash recycling occurs naturally in wet bottom furnaces, attempts have been made to use the same mechanism to recycle collected fly ash.
Fly ash is normally collected in electrostatic precipitators, baghouses or other suitable devices. The collected fly ash may thus be blown back into the furnace. In this case, the recycling improves efficiency by burning the carbon. To the extent that the recycled ash returns as fly ash, the process lowers the percent carbon in the fly ash, improving its marketability. To the extent the ash melts and flows from the furnace, the amount of fly ash which must be removed is reduced.
This technique does not result in the melting of all the recycled fly ash. Some of the recycled fly ash is blown back through the boiler and is once again collected in the particulate control equipment. Consequently, there is a need for a system which collects the fly ash so it can be melted and discharged as slag.